Charged particle beam apparatuses are used in a plurality of industrial fields. Inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems are only some of these fields.
In general, there is a high demand for structuring and inspecting specimens within the micrometer or nanometer scale. On such a small scale, process control, inspection or structuring is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
Generally, charged particle beam devices are operated under vacuum conditions to avoid, e.g. ionization of surrounding gases. In spite of that, electrons impinging on component surfaces of the device, like extractors, anodes, apertures or the chamber wall result in an emission of contaminants. Thus, a shower of residual gas is generated. The residual gas contains molecules which can be hit by electrons. Thereby, ions, ionized molecules and other particles can be created. In the case of ions and ionized molecules having a charge which is opposite to the charge of the charged particles emitted by an emitter, the ions and ionized molecules in the residual gas are accelerated towards the emitter. As a result, the emitter can be mechanically deformed from the impingement of the ions and ionized molecules or these particles can be deposited on the emitter. Thus, emitter noise is introduced.
The above described contaminations are even more critical for cold field emitters or thermally assisted cold field emitters (CFE), because such emitters have an even higher need to have a high vacuum and a clean environment. Even though such emitters are known for a long time, implementation thereof in industrial applications, wherein a high availability of systems and high stability due to desired automation is demanded, have been experienced to be difficult due to these requirements. Thus, it is difficult to provide a high brightness electron source for electron microscopy, such as a CFE or thermally assisted CFE, and more specifically for electron beam based wafer inspection, with sufficient stability of operation.